provide more errors for the "merge into empty head" case

provide more errors for the "merge into empty head" case

A squash merge into an unborn branch could be implemented by building the
index from the merged-from branch, and doing a single commit, but this is
not supported yet.

A non-fast-forward merge into an unborn branch does not make any sense,
because you cannot make a merge commit if you don't have a commit to use
as the parent.

Signed-off-by: Paolo Bonzini <redacted>
Signed-off-by: Junio C Hamano <redacted>